[cfe-dev] [RFC] Adding a different mode of "where clang spends time" reporting (timeline/flamegraph style)

Reid Kleckner via cfe-dev cfe-dev at lists.llvm.org
Tue Jan 22 17:00:00 PST 2019

This is pretty cool!

I think the question is, "what next"? I don't really want to say "merge it
right away!", because then we will have two systems for generating time
reports, but I also don't want to send you off into the hills to refactor
the existing Timer to support both output formats, and then decide what to
do with all the fine-grained timers geared towards compiler developers.
Would you be interested in working in that that direction, or would you
rather hand the code off to someone else to try to integrate it more

I feel like right now C++ users don't have a lot of visibility into build
performance, so it ends up falling to specialist toolchain people to go and
debug these problems. If we made it easy to see into the performance of
compilation, it would democratize build time optimization, letting
generalists do these kinds of codebase cleanups. At least, I hope it will.

On Sun, Jan 13, 2019 at 4:31 AM Aras Pranckevicius via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello!
> TL;DR: I have made a Clang/LLVM code change that adds "-ftime-trace"
> option, that produces Chrome Tracing format output. Would like comments on
> whether that is a good idea or not, or perhaps someone else is already
> doing this. My current (WIP) patch in github PR format is here
> https://github.com/aras-p/llvm-project-20170507/pull/2 -- with images and
> trace output files attached.
> Longer version:
> Current implementation of "-ftime-report" has several issues, particularly
> when I'm just a "user" of the compiler:
> - it outputs a lot of information (almost half of it is duplicated since
> clang 7.0),
> - a lot of that information is things that only compiler developers would
> know about,
> - has quite large overhead, I've seen it make compile times take 1.5x
> longer,
> - has very little information about "frontend" part (preprocessing,
> parsing, instantiation, C++ modules),
> - the things it reports are only "summaries", i.e. "how much time it took
> to do work X in total". e.g. it can tell that "inlining all functions took
> X seconds", but in case there was just one super slow function to inline
> among
> thousands, it will not tell which one was the slow one.
> I have written a blog post about this (as well as lack of "good" time
> reporting tools in Visual Studio and gcc) recently,
> http://aras-p.info/blog/2019/01/12/Investigating-compile-times-and-Clang-ftime-report/
> At work (Unity game engine), with codebase of similar size to whole of
> Clang/LLVM (2-3 million lines of code), we had really good experience in
> adding timeline/flamegraph visualizations to various parts of our "build
> system". This can tell us which .cpp files were slowest to compile in the
> whole build, but I also wanted similar tooling for things "inside" single
> .cpp file compilation.
> Thus this attempt at adding a new time trace profiling mode.
> I have current changes on github here,
> https://github.com/aras-p/llvm-project-20170507/pull/2 -- can do a proper
> "patch" thing via Phabricator if needed.
> My current code change does not quite match Clang/LLVM code standards and
> probably needs some work, but the general approach seems to work. Already
> found one case of Clang being very slow at parsing some weird recursive
> macro thingamabob that we had; was causing about 5-8 seconds just to
> include one header file. I probably would have never found it without this
> type of visualization. Here it is very clear that among all the things,
> parsing just that one header file takes almost all the time:
> https://user-images.githubusercontent.com/348087/51038295-76efb780-15bb-11e9-926f-a6be1ffd03f1.png
> Regards,
> --
> Aras Pranckevičius
> work: http://unity3d.com
> home: http://aras-p.info
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
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